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Are your aeration basin D.O. meters telling you the whole D.O. story?

11/18/2021

D.O. meters report dissolved oxygen in the water phase of the aeration basin. We know that in a flask, obligate aerobes such as AOB do very well at 2.0 mg/L DO. Why is it that in many aeration basins the DO must be at least 3.5 mg/L for proper obligate aerobic organism growth?

I have seen two reasons why you may need to run higher than textbook D.O. residuals.

DO probe placement – a single probe can have sampling error and can easily be checked by testing other points in the basin with a portable meter.
DO in floc is different than in the water phase (where the probe is located) - floc is composed of microbes, EPS, non-soluble organics, and inorganics. DO must penetrate the floc to reach microbes deep inside the floc. Larger, higher EPS flocs can have anoxic/anaerobic zones inside.

Do you have any other ideas why many systems require operation above 2.0 mg/L D.O. residuals in the aeration basin?

Aeration - what is the best way to supply oxygen to aerobic microorganisms

11/16/2021

Aerobic microbes rely on oxygen to carry out their life functions. Oxygen, being only moderately soluble in water, must be constantly added to aerobic wastewater treatment systems to ensure proper waste treatment. Aeration energy use is one of the largest costs in running a treatment system, so finding the best aeration type is a key part of running and maintaining a system.

How oxygen is transferred into water
Oxygen dissolves into water at the air/water interface. Oxygen from the atmosphere (20% O2) naturally enters water at the surface. In most aerobic treatment systems oxygen transfer is done via air bubbles passing through the water (diffuser or jet type aeration) or through the mechanical action of surface aerators. Key is the surface to volume of the bubble - the smaller the bubble the higher ratio of surface area for diffusion compared to larger or coarse bubbles.

What we want aeration systems to do

Provide efficient DO transfer
Keep biological solids suspended
Provide sufficient mixing to avoid short circuiting or dead zones
Problem is you cannot have everything!
Best solution may be combining multiple types into a hybrid system

Aster Bio's new Environmental Genomics™ sample prep robot!

11/9/2021

Aster Bio uses molecular testing including Next Generation Sequencing (NGS) and qPCR to monitor the microbial populations in wastewater and other environmental samples. With both technologies, samples require DNA extraction and purification before the actual testing starts. To speed up sample processing times, Aster Bio has invested in a new robot to process samples in what was a time-consuming, labor-intensive step.

With increased automation, we will be able to increase our sample throughput and give rapid results on microbial population composition to our clients. The most common applications of Enviromental Genomics™ testing include:

Microbial Community Analysis (MCA) - a total microbial census of all bacteria present with information on functions or physical characteristics such as Ammonia Oxidation, Phosphate Removal, Bulking, Foaming, or Sulfur Reducing/oxidizing.
AOB/NOB qPCR - with in-house developed primers, Aster Bio evaluates ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) in wastewater samples. The qPCR results provide information well before effluent ammonia or nitrite residuals change.
Bulking & specific filament - often customers find that key bulking or filamentous organisms appear during MCA testing. Aster Bio is now able to build rapid, accurate qPCR tests for monitoring these specific organisms with results being available in hours versus days.

Making sense of OUR tests

11/3/2021

We often run OUR tests and record the oxygen uptake of a sample as mg O2/L/hr. The key is to make sense of the respiration rate. First, unless you run a constant MLSS or MLVSS, you should divide the OUR by MLSS in grams to get the SOUR which helps account for background solids impact on respiration. Now with the OUR or SOUR numbers, you need to think about what the test is telling you about microbial health.

Sudden increase in OUR is normally see with high soluble BOD (organic) loadings. If you run ATP tests, you should see an increase in free ATP (ATP in solution) which indicates rapid cell division or a move to the left on the growth curve.
A slow increase in OUR numbers can be from changes in total biological solids which is why SOUR calculation is important. I have also seen increases in VSS relative to MLSS create a change in OUR numbers. Other factors can be "fat slduge" which is sludge with adsorbed insoluble organics or high EPS which is simply food in storage.
What about a sudden drop in OUR numbers. Not often seen in domestic wastewater, shock or toxic loadings can cause a sudden drop in biological activity or respiration rates. In this case, you will see a drop in OUR and other signs of stress such as deflocculation, increased turbidity, loss of indicator protozoa, and loss of nitrification. If the shock was due to a short term spill, you will see the low OUR replaced by a rapid increase in OUR as the surviving bacteria begin to multiply in log growth.

I recommend daily running of OUR. This test uses exisiting equipment with no reagents or excessive time requirements. You can also use influente spiked OUR to predict toxicity or biomass inhibition. Key thing is to run the test frequently so you know what the normal OUR is for the system and enable you to see any significant change in microbial activity.

Are your aeration basin D.O. meters telling you the whole D.O. story?

Aeration - what is the best way to supply oxygen to aerobic microorganisms

Aster Bio's new Environmental Genomics™ sample prep robot!

Making sense of OUR tests

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